The present invention relates to piezoelectric/electrostrictive actuators and methods for manufacturing the same. More particularly, it relates to a piezoelectric/electrostrictive film-type actuator which is used for a dislocation control device, a solid-state device motor, an ink-jet head, a relay, a switch, a shutter, a pump, a fin, and so on, which operates in response to a dislocation of an element and which serves as a transducer for converting mechanical energy into and from electrical energy, so as to achieve a quicker response, a higher energy conversion efficiency, and a larger bending dislocation, and it relates to methods for manufacturing the piezoelectric/electrostrictive actuator.
Piezoelectric/electrostrictive actuators which serve as a mechanism for increasing a pressure in a pressurized chamber formed in a base of the actuator and which change the volume of the pressurized chamber in response to a dislocation of a piezoelectric/electrostrictive element disposed on a wall of the pressurized chamber have been recently known. Such piezoelectric/electrostrictive actuators are used for, for example, an ink pump of a print head of an ink-jet printer and the like, for discharging an ink particle (ink droplet) from a nozzle communicating with the pressurized chamber by increasing the pressure in the pressurized chamber filled with ink in response to a dislocation of the piezoelectric/electrostrictive element, and thus for performing printing.
An example of an ink-jet print head using piezoelectric/electrostrictive actuators as shown in FIGS. 4 and 5 is disclosed in JP-A-06-40035.
An ink-jet print head 140 has an ink nozzle member 142 and a piezoelectric/electrostrictive actuator 145 integrally bonded with the nozzle member, and has a configuration in which ink fed in cavities 146 formed in the piezoelectric/electrostrictive actuator 145 is discharged from nozzles 154 formed in the ink nozzle member 142.
More particularly, the piezoelectric/electrostrictive actuator 145 has a ceramic base 144 and piezoelectric/electrostrictive elements 178 integrally formed with the ceramic base 144. The ceramic base 144 has a closing plate 166, a connecting plate 168, and a spacer plate 170 interposed between the closing plate and the connecting plate, these plates having a thin flat shape and being integrally formed.
The connecting plate 168 has first communication openings 172 and second communication openings 174 formed at positions corresponding to communication holes 156 and orifices 158, respectively, formed in an orifice plate 150. While the first communication opening 172 has substantially the same or a little larger inner diameter than that of the communication hole 156, the second communication opening 174 has a larger diameter than that of the orifice 158 by a predetermined amount.
Also, the spacer plate 170 has a plurality of long rectangular windows 176 formed therein. The spacer plate 170 is overlaid on the connecting plate 168 so that one of the first communication openings 172 and one of the second communication openings 174 formed in the connecting plate 168 are opened to the corresponding window 176.
Furthermore, the spacer plate 70 has the closing plate 166 and the connecting plate 168 overlaid on the respective surfaces thereof so that the closing plate 166 covers the windows 176. Thus, the ceramic base 144 has the cavities 146 formed therein which communicate with the outside via the first and second communication openings 172 and 174.
In such a piezoelectric/electrostrictive film-type actuator 145, in order to provide a larger dislocation so as to discharge a larger droplet, it is effective to make the closing plate 166 serving as upper walls as well as diaphragms of the cavities 146 thinner and also the short sides of the rectangular cavities 146 wider; however, this configuration leads to a decrease in the stiffness of the closing plate 166, resulting in a deterioration in the quick response of the actuator 145.
In order to increase the stiffness so as to achieve a quicker response, it is effective to make the closing plate 166 thicker and also the short sides of the long rectangular windows 176 (cavities 146) shorter; however, making the closing plate 166 thicker leads to thicker diaphragms, resulting in a small dislocation of the diaphragms, thereby causing a problem in that a required volume of a droplet is not discharged. In other words, it is difficult to achieve a large dislocation and a quick response, at the same time, of the piezoelectric/electrostrictive actuators by only optimizing the dimensions of the actuators when further improved performances of the actuators are required.
To solve these problems, the same applicant has proposed a piezoelectric/electrostrictive film-type actuator, in PCT Application No. PCT/JP02/02290, wherein piezoelectric/electrostrictive elements, each having a plurality of layers of piezoelectric/electrostrictive films and electrode films laminated therein, are disposed on a base. The proposed actuator is the same as a piezoelectric/electrostrictive film-type actuator 71, shown in FIG. 7, wherein a piezoelectric/electrostrictive element 78 having electrode films 73, 75, and 77 and a plurality of (i.e., two-layered) piezoelectric/electrostrictive films 79 laminated therein is disposed on a ceramic base 44 having a cavity 46 therein. When compared to a piezoelectric/electrostrictive element having a single-layered piezoelectric/electrostrictive film, the piezoelectric/electrostrictive element 78 increases a response speed because of its higher stiffness and also produces a larger force as a whole since the element 78 is driven by the plurality of piezoelectric/electrostrictive films, thereby achieving a relatively large dislocation despite its high stiffness. As a result, when the actuator is applied, for example, to an ink-jet print head, the actuator discharges a required volume of a droplet more quickly.
It is an object of the present invention to fully complement the foregoing proposal. That is to say, it has been found that when the proposed piezoelectric/electrostrictive element having a plurality of layers of piezoelectric/electrostrictive films and electrode films laminated therein is manufactured by firing all together after the piezoelectric/electrostrictive films and the electrode films are laminated, the upper surface of the piezoelectric/electrostrictive films, i.e., the piezoelectric/electrostrictive film in the uppermost layer is likely to be partially decomposed in firing, thereby causing different phases such as decomposed portions 80 illustrated in the piezoelectric/electrostrictive film-type actuator 71 shown in FIG. 7 to be produced, and leading to the likelihood of withstand voltage deterioration.
More particularly, for example, in PZT typically used as a piezoelectric material, Pb acting as a component of PZT and having a high vapor pressure property evaporates in firing, and thus a PZT crystal is decomposed, resulting in crater-like traces in which glass-like material (different from PZT) mainly including Zr and Ti resides. Since these portions have a reduced thickness of the piezoelectric/electrostrictive film and contain substances having different dielectric constants, the element is likely to have an electric field concentration during polarization or when a driving voltage is applied, thereby causing an electrical breakdown, that is, causing a problem of a reduced withstand voltage.
It has also been found that the piezoelectric/electrostrictive film in the lowermost layer closest to the ceramic base experiences an anti-shrinkage resistance most from the ceramic base (i.e., a closing plate) in firing shrinkage, and also experiences a heat stress most from the ceramic base (i.e., the closing plate) due to a difference in thermal expansion and shrinkage in cooling down after firing, thereby preventing the film in the lowermost layer from achieving its primary piezoelectric performance, and causing the film to have a reduced bending dislocation. The present invention is made in view of these problems.
Accordingly, it is an object of the present invention to solve the above described problems, in other words, to provide a piezoelectric/electrostrictive film-type actuator which solves the problem of the likelihood of withstand voltage deterioration, and which effectively achieves a bending dislocation. Research focusing on the thickness of a plurality of piezoelectric/electrostrictive films constituting a piezoelectric/electrostrictive element has been conducted in order to solve the above problems and revealed that the above object can be achieved by the following means.
More particularly, the present invention provides a piezoelectric/electrostrictive film-type actuator which comprises a ceramic base and a piezoelectric/electrostrictive element disposed on the ceramic base, the element comprising piezoelectric/electrostrictive films and electrode films, the actuator being driven in accordance with a dislocation of the piezoelectric/electrostrictive element, characterized in that the piezoelectric/electrostrictive films and the electrode films are alternately laminated so as to construct the uppermost layer and the lowermost layer with the electrode films in the piezoelectric/electrostrictive element, and the piezoelectric/electrostrictive films have two layers and no pores, containing a different phase formed by a decomposed material thereof, in the boundary sandwiched therebetween, and the upper layer of the two-layered piezoelectric/electrostrictive films is thicker than the lower layer thereof.
In the present invention, the thickness tU of the upper-layered piezoelectric/electrostrictive film and the thickness tB of the lower-layered piezoelectric/electrostrictive film preferably satisfy at least one of the following expressions:
tUxe2x89xa7tBxc3x971.1xe2x80x83xe2x80x83(Numerical Expression 1) 
tUxe2x89xa7tB+1 (xcexcm)xe2x80x83xe2x80x83(Numerical Expression 2). 
The piezoelectric/electrostrictive film-type actuator according to the present invention may have a structure in which the ceramic base has a cavity formed therein so as to be pressurized by deforming a diaphragm (i.e., the upper wall of the cavity) bonded to the piezoelectric/electrostrictive element in accordance with a dislocation of the piezoelectric/electrostrictive element. In this case, the thickness tW of the diaphragm, the thickness tU of the upper-layered piezoelectric/electrostrictive film, and the thickness tB of the lower-layered piezoelectric/electrostrictive film preferably satisfy the following expression (Numerical Expression 3), and the thickness tW of the diaphragm is preferably less than or equal to 50 xcexcm, more preferably from 3 to 12 xcexcm:
tU+tBxe2x89xa72xc3x97tWxe2x80x83xe2x80x83(Numerical Expression 3). 
The above-described piezoelectric/electrostrictive film-type actuator according to the present invention is suitably applied to an ink pump of an print head of an inkjet printer.
Additionally, in the piezoelectric/electrostrictive film-type actuator according to the present invention, the thickness of each layer of the upper-layered and lower-layered piezoelectric/electrostrictive films is preferably less than or equal to 15 xcexcm, and more preferably from 3 to 10 xcexcm. Also, at least one layer of the piezoelectric/electrostrictive films is more preferably formed by an electrophoretic method. Furthermore, at least two piezoelectric/electrostrictive elements are preferably disposed on a single ceramic base.
Moreover, when the piezoelectric/electrostrictive film-type actuator has a structure in which the ceramic base has the cavity formed therein so that the piezoelectric/electrostrictive element deforms the diaphragm so as to pressurize the cavity as described above, the ceramic base is preferably formed by integrally laminating a plurality of thin plates, and more preferably formed by integrally laminating two or three thin plates.
Next, the present invention provides a method for manufacturing a piezoelectric/electrostrictive film-type actuator, wherein the actuator has a ceramic base and a piezoelectric/electrostrictive element disposed on the ceramic base, the element has piezoelectric/electrostrictive films and electrode films, and the ceramic base has a cavity formed therein so as to be pressurized by deforming a diaphragm bonded to the piezoelectric/electrostrictive element in accordance with a dislocation of the piezoelectric/electrostrictive element. The method comprises a step A for forming a ceramic laminate by preparing at least one green sheet and one or more other green sheets having at least one hole formed therein, by laminating these sheets so as to form a green sheet laminate such that said at least one green sheet having no hole serves as the upper surface of the green sheet laminate, and by firing the green sheet laminate; a step B for forming the lower electrode film on the upper surface of the obtained ceramic laminate by a film forming method and for firing the lower electrode film; a step C for forming the lower-layered piezoelectric/electrostrictive film on the lower electrode film by the film forming method, for forming the middle electrode film on the lower-layered piezoelectric/electrostrictive film by the film forming method, and for forming the upper-layered piezoelectric/electrostrictive film, which is thicker than the lower-layered piezoelectric/electrostrictive film, on the middle electrode film by the film forming method; a step D for firing the laminated piezoelectric/electrostrictive films and the middle electrode film all together; and a step E for forming the upper electrode film on the upper-layered piezoelectric/electrostrictive film by the film forming method and for firing the upper electrode film.
In the method for manufacturing a piezoelectric/electrostrictive film-type actuator according the present invention, the thickness tU of the upper-layered piezoelectric/electrostrictive film and the thickness tB of the lower-layered piezoelectric/electrostrictive film preferably satisfy the foregoing expressions (i.e., at least one of Numerical Expressions 1 and 2).
Also, each layer of the piezoelectric/electrostrictive films and electrode films may be formed by applying the film forming method a plurality of times. The film forming method may be at least one thick film forming method selected from the group consisting of a screen printing method, a dipping method, a coating method, and an electrophoretic method. For example, the film forming method for the piezoelectric/electrostrictive films may comprise the screen printing method for the first time of film forming and the electrophoretic method for the following times of film forming.
The piezoelectric/electrostrictive film-type actuator manufactured by the method according to the present invention is suitably applied to an ink pump of a print head of an ink-jet printer.